Plate Tectonics And Plate Boundaries HORRAH!!!
History of Plate Tectonics and Alfred Wegener
In the 1500 s, a Dutch mapmaker, Abraham Ortelius noticed the continents across the Atlantic Ocean fit like puzzle pieces. He proposed that North and South America had been separated from Europe and Africa by earthquakes and floods. History
Abraham Ortelius Map
Edward Suess An Austrian geologist, in the late 1800 s, proposed the southern continents had once been joined together in a single landmass He named this landmass Gondwanaland.
Gondwanaland
In 1912 a German meteorologist, Alfred Wegener, had an idea about continental movement. His hypothesis was called Continental Drift. Alfred Wegener
His hypothesis proposed that the continent s had once been joined as a single landmass, called PANGAEA. Pangaea is a greek word meaning All the Earth
PANGAEA Wegener believed that Pangaea began to break apart 250 million years ago. The continents have been continuously moving since then to where they are now.
Wegener s Evidence 1. Rocks Believed there should be similar rock types on opposite sides of the Atlantic Appalachian Mtns. are similar to the Scottish Highlands Rocks in Brazil matched those in Africa (similar age of 2.2 billion years old)
More Evidence 2. Fossils Animal Fossils Lystrosaurus land reptile Mesosaurus freshwater reptile Cynognathus land reptile ALL couldn t have swam across Atlantic Ocean (salt water)
Fossil Evidence Plants Glossopteris Animals Seeds too heavy to blow across ocean Too fragile to float across ocean Land animals couldn t swim Freshwater animals can t swim in ocean (salt water) Couldn t have evolved the same way in different locations!
Even More Evidence 3. Climate Coal deposits found in Antarctica Glacier deposits found in Africa, India, Australia, and South America
Science Haters Other scientists rejected his hypothesis because he couldn t explain what was causing the continents to move (a.k.a. HOW?) He thought rotation of the Earth but physicists said no Also thought continents plowed through ocean floor but there was no evidence for that.
Arthur Holmes 1929, a geoscientist, Holmes proposed that convection occurred in the mantle. This convection could push and pull plates (continents) apart or together. But NO evidence!!!
FINDING EVIDENCE The first way to map the ocean floor used Sonar. This way stunk because it didn t accurately map the floor.
The then invented the Magnetometer. This could detect small changes in the magnetic field in rocks and map them.
Seafloor Spreading Mid-Atlantic Ridge
Seafloor Spreading new ocean crust is formed at ocean ridges and destroyed at deep sea trenches.
Harry Hess Used paleomagnetism and seafloor mapping to prove convection is a source for continental drift
Convection current/plate motion http://news.bbc.co.uk/nol/shared/spl/hi/pop_ups/04/world_how_volca noes_erupt/img/1.jpg Plate motion is thought to be caused by convection currents in the asthenosphere. Since the hotter material deep in the asthenosphere is less dense it will slowly rise, as it reaches the base of the lithosphere it begins to cool, become more dense, and will sink. Rising convection currents will move plates apart (divergent), while sinking convection currents will move plates together (convergent).
Plate Boundaries and Convection Currents
These new maps showed the different magnetism in different layers of rock. As new crust is created the iron bearing minerals point to where the magnetic pole is at the time of creation. This is called Paleomagnetism
Mid-Ocean Ridge Isochron line of same aged rock
Magnetic Pole Reversals Random reversal of Earth s Polarity Average irregular 300,000 year cycles Last one was over 780,000 years ago = overdue Unknown as to why Maybe external, maybe internal
This proved that new sea floor was being created at mid-ocean ridges. Identical magnetism on opposite sides of ridges. Thus, the plates where moving.
Plate Boundaries and Convection Currents
Types of Boundaries
Divergent Boundaries two plates move apart
Ocean/Ocean Formations- ocean ridge, fissures
Mid-Atlantic Ridge it is 40,000 miles long
Mid-Ocean Ridge Magnetic Reversals
Land/land Formations rift valley
Great African Rift Valley
Video Website - Divergent http://www.absorblearning.com/media/item. action?quick=12n
Convergent two plates come together
Land/ocean Formations- subduction zone, trench, volcanoes, earthquakes
subduction Video - Subduction
Mt St. Helens
Ring of Fire
Land/land Formations Large Mountains (Orogeny)
Video Convergent Mtns. converging mts
Himalayas
Ocean/ocean Formations island arc, trench
Eastern Caribbean
Video Convergent - Island Arc island arc
Transform the plates slide past each other, they form faults
Friction builds up between the plates until they snap and cause EARTHQUAKES
San Andreas Fault
Hotspot
Hotspots The hotspot is also known as a magma chamber, it is located in the mantle
The Hotspot then explodes, creating a volcano
The plate then moves. Notice the volcano moves with the plate, but the hotspot stays were it was
The Hotspot then explodes again, forming a new volcano
The plate moves again, starting the process over
Another volcano is then created forming a Volcanic Chain
An example of a hotspot is Hawaii